1. Field of the Invention
The present invention relates to a sheet processing apparatus including an aligning device that aligns sheets stacked in a stacking device, and an image forming apparatus provided with the sheet processing apparatus.
2. Description of the Related Art
Conventionally, a sheet processing apparatus has been proposed which discharges sheets sheet by sheet conveyed from an image forming apparatus such as a copying machine or a printer onto a processing tray by a discharging device, aligns the sheets using aligning plates each time one sheet is discharged onto the processing tray, staples a bundle of sheets of one copy when they have been discharged onto the processing tray, and discharges the bundle of sheets onto a stack tray via a pair of bundle discharging rollers.
However, assuming that an image forming apparatus is used which discharges sheets at a higher speed and at shorter time intervals (i.e., shorter sheet conveyance time intervals) than a conventional image forming apparatus, in order to improve the productivity, there is a limitation on a period of time required for sheet alignment such that a sheet is discharged onto thee processing tray before alignment of a preceding sheet is completed. For this reason, the sheet conveyance intervals must be extended, which lowers the productivity of the image forming apparatus.
Also, when sheets are simply discharged onto a discharged sheet tray, if the sheet conveying speed is reduced in the sheet processing apparatus in the case where sheets are conveyed at a high speed and at short intervals, a sheet is discharged onto the tray while a preceding sheet is being aligned, and hence it is impossible to reduce the sheet conveying speed so as to adjust the sheet jumping amount (the degree to which a discharge. Therefore, reliable sheet alignment cannot be realized, and the productivity of the image forming apparatus can be deteriorated due to increase in sheet conveyance time intervals.
Further, if a motor which enables high-speed sheet alignment is adopted so as to convey sheets at a high speed and at short intervals and align the sheets each time one sheet is discharged onto the processing tray, the cost and the size of the motor will increase.